Fuel cells are designed to cause an electrochemical reaction between a fuel gas containing hydrogen and an oxidizing gas containing oxygen such as air, thereby simultaneously generating electric power and heat. As an example of fuel cells, there are known polymer electrolyte fuel cells having the following structure.
A catalytic reaction layer containing carbon powder which carries a platinum group metal catalyst as a chief component is formed on both sides of a polymer electrolyte membrane, for selectively transporting hydrogen ions. On the outer face of each catalytic reaction layer, a diffusion layer is formed which is made of, for example, carbon paper or carbon cloth having both fuel gas permeability and electronic conductive properties. These diffusion layers and catalytic reaction layers are combined thereby forming a membrane electrode assembly.
For power collection, this membrane electrode assembly is sandwiched by separators made from conductive materials such as glass-like carbon or metal. These separators are provided with gas passages that are formed so as to expose the aforesaid fuel gas and oxidizing gas to the membrane electrode assembly and with cooling fluid passages that are formed so as to control the temperature of the membrane electrode assembly, in other words, so as to recover the heat generated together with electric power. Generally, a gas sealing material or gasket is provided between the membrane electrode assembly and each separator, for preventing leakage of the supplied fuel gas and oxidizing gas to the outside and mingling of these gases.
The membrane electrode assembly sandwiched between the separators serves as a basic unit. In a polymer electrolyte fuel cell, such membrane electrode assemblies are stacked in numbers corresponding to the design output of electric power or heat of the polymer electrolyte fuel cell.
In a fuel cell system, a fuel gas feeder for supplying the fuel gas and an oxidizing gas feeder for supplying the oxidizing gas are connected to the polymer electrolyte fuel cell (hereinafter abbreviated as PEFC). In addition, there are provided, according to need, an exhaust heat recovery system for recovering generated heat and a power converting system for making the electric power generated in the PEFC usable. Further, a control unit for controlling these systems is provided.
The fuel gas feeder includes a hydrogen generator for generating a hydrogen-rich gas (i.e., the fuel gas) by reforming hydrocarbon fuel such as natural gas, propane gas and gasoline to output to the PEFC. The oxidizing gas feeder consists of, for example, a blower or fan and supplies air to the PEFC as the oxidizing gas. In some cases, the fuel gas feeder and oxidizing gas feeder are equipped with a humidifier for controlling the amount of moisture contained in the fuel gas or oxidizing gas to be supplied to the PEFC. The exhaust heat recovery system is composed of a heat exchanger and a hot water tank. The heat exchanger recovers the heat retained by, for instance, the cooling fluid flowing in the cooling fluid passage by means of water to produce hot water. The hot water tank stores this hot water. The power converting system includes an inverter for converting a dc power generated by the PEFC into an ac power and a transformer.
As described earlier, within the PEFC, the fuel gas or oxidizing gas flows in the route made by the gas passages formed in the gasket, polymer electrolyte membrane and separators. However, the constituents of the gas passages degrade causing, for instance, increases in the gas permeation of the polymer electrolyte membrane and hardening of the gasket, so that the airtightness of the gas passages decreases. This entails leakage of the fuel gas or oxidizing gas to the outside or mingling of these gases. The leakage of the fuel gas to the outside and the mingling of the fuel gas/the oxidizing gas may trigger off abnormal combustion or an explosion. Even if they do not result in abnormal combustion or an explosion, the leakage to the outside causes an insufficient supply of the fuel gas or oxidizing gas to the catalyst reaction layer and, in consequence, insufficient exposure of the gas to the membrane electrode assembly. As a result, the polarization resistance of the electrode reaction increases with a decrease in the output of the PEFC.
To prevent damage to the PEFC and a decrease in the performance of the fuel cell, a fuel cell system or detection method is required which enables detection of the airtightness of the passages in which the fuel gas or oxidizing gas flows. General type pressure vessels usually employ “escape probability detection” in which pressure gas is sealed in a vessel and the time taken for pressure to decrease or a decrease in pressure within a specified period of time is detected. However, the detection method, in which pressurized gas is sealed in a fuel cell system and the progress of decreasing of pressure is observed, can not be practically applied to fuel cell systems, because they are operated as needed and therefore detection of the airtightness of the passages for the fuel gas or oxidizing gas in the PEFC has to be promptly performed so as not to hinder the operation of the PEFC. Apart from the above method, there have been heretofore proposed several fuel cell systems and methods for detecting the airtightness of a fuel cell. Typical techniques are as follows.
In the fuel cell system disclosed in Patent Document 1, the consumption of the fuel gas is calculated based on the output current of the fuel cell and the pressure of the fuel gas within the fuel gas cylinder is calculated from the fuel gas consumption. Then, the presence/absence of fuel gas leakage is determined from a comparison between this calculated pressure value and a detected pressure value that is obtained from actual detection with a pressure sensor.
According to the diagnosis method disclosed in Patent Document 2, a hydrogen-containing gas and an oxygen-containing gas are supplied to the fuel electrode and oxidant electrode, respectively, of the fuel cell and a rapid change in the generated voltage of the fuel cell caused by a decrease in the supply of the oxygen-containing gas is detected. Then, the leakage of hydrogen in the fuel cell is calculated from the relationship between the oxygen-containing gas and the generated voltage.
There have been proposed fuel cell systems that make a judgment on deterioration of a fuel cell. According to this technique, the condition of a fuel cell is detected in various ways thereby determining whether or not the fuel cell has deteriorated and the result of the determination is fed back to the control mechanism for the fuel cell to restrain the progression of the deterioration so that the durability and service life of the fuel cell and the fuel cell system are increased.
For instance, Patent Document 3 discloses a PEFC operating method according to which a judgment is made to check whether the operational state of the PEFC is in a performance decreasing zone by analyzing impurity ions contained in the moisture of a fuel gas humidifying water or the like from the PEFC. If it is determined that the operational state is in the performance decreasing zone, the operation of the PEFC is brought to a stop or operating conditions for the PEFC are limited, thereby making the operational state of the PEFC get out of the performance decreasing zone.
Patent Document 4 discloses a method of estimating the service life of a fuel cell. According to this method, a fuel cell is operated in several basic operating patterns and its service life is estimated based on the time taken for power generation and the change rate of output voltage in each basic operation pattern.    Patent Document 1: Japanese Laid-Open Patent Application Publication No. Hei 11-224681    Patent Document 2: Japanese Laid-Open Patent Application Publication No. Hei 9-27336    Patent Document 3: Japanese Laid-Open Patent Application Publication No. 2004-127548    Patent Document 4: Japanese Laid-Open Patent Application Publication No. Hei 11-97049